A long-standing problem in tumor immunology that poses a serious challenge for cancer immunotherapy is why tumor-killing CD8+ T cells do not efficiently infiltrate tumors. In stark contrast, CD4+ T cells, especially regulatory T cells, which induce immunosuppression and promote tumor growth and metastasis, accumulate in tumors. Extensive studies from our laboratory and other groups show that STAT3, a Signal Transducer and Activator of Transcription family protein critical for tumor cell survival and invasion, mediates the crosstalk between tumor cells and various immune cells, causing tumor immunosuppression. Over the last five years, our work further establishes a role of STAT3 in impacting both CD8+ and CD4+ T cells in tumor-bearing hosts. Our results suggest that STAT3 activity within CD4+ T cells is critical for their tumor accumulation. By contrast, STAT3 intrinsic to CD8+ T cells inhibits their tumor infiltration. Based on these findings, we hypothesize that tumor recruitment of CD4+ and CD8+ T cells utilizes distinct signaling pathways/factors, resulting in opposing biological functions that enhance tumor progression. In this application, we will test our hypothesis by examining whether signaling of sphingosine-1-phosphate (S1P) and its receptor, S1PR1, which we have demonstrated to be critical for persistent STAT3 activation in tumor cells and tumor-associated immune cells, is essential for CD4+ T cell mobilization to tumor sites. We will also assess whether STAT3 inhibition-induced expression of interferon (IFN) and T cell attractants, also known as chemokines, causes CD8+ T cell tumor infiltration. Moreover, we propose to dissect out detailed molecular mechanisms by which S1P/S1PR1-STAT3 signaling and STAT3 inhibition-induced IFN/chemokine signaling modulate CD4+ and CD8+ T cell mobilization to tumor sites, respectively. Results from our proposed studies will likely generate new knowledge on fundamental mechanisms underlying the imbalance of CD8+ and CD4+ T cells in tumor-bearing hosts, as well as identify new targets to potentially develop paradigm-shifting cancer immunotherapeutic approaches.